Liquid crystal displays (LCDs) are passive flat panel displays which depend upon external sources of light for illumination. They are manufactured as segmented displays or in one of two basic configurations. The substrate needs (other than being transparent and capable of withstanding the chemical conditions to which it is exposed during display processing) of the two matrix types vary. The first type is intrinsic matrix addressed, relying upon the threshold properties of the liquid crystal material. The second is extrinsic matrix or active matrix (AM) addressed, in which an array of diodes, metal-insulator-metal (MIM) devices, or thin film transistors (TFTs) supplies an electronic switch to each pixel. In both cases, two sheets of glass form the structure of the display. The separation between the two sheets is the critical gap dimension, of the order of 5-10 μm.
Intrinsically addressed LCD's are fabricated using metal deposition techniques, typically at temperatures ≦350° C., followed by standard metal etching procedures. As a result, the substrate requirements therefor are often the same as those for segmented displays. Soda-lime-silica glass with a barrier layer has proven to be adequate for most needs. A high performance version of intrinsically addressed LCDs, the super twisted nematic (STN) type, has an added requirement of extremely precise flatness for the purpose of holding the gap dimensions uniform.
Extrinsically addressed LCD's can be further subdivided depending upon the nature of the electrical switch located at each optical element (subpixel). Two of the most popular types of extrinsically (or active matrix, AMLCD) addressed LCD's are those based on either amorphous (a-Si) or poly-crystalline (poly-Si) silicon thin film transistors (TFT's).
U.S. Pat. No. 4,824,808 (Dumbaugh, Jr.) lists four desirable properties for a glass to exhibit in order to fully satisfy the needs of a substrate for extrinsically addressed LCD's:
First, that the glass be essentially free of intentionally added alkali metal oxide to avoid the possibility of alkali metal contamination of the TFT;
Second, that the glass be sufficiently chemically durable to withstand the reagents used during the manufacture of the TFT;
Third, that the expansion mismatch between the glass and the silicon present in the TFT array be maintained at a relatively low level even as processing temperatures for the substrates increase; and
Fourth, that the glass be capable of being produced in high quality thin sheet form at low cost; that is, it must not require extensive grinding and polishing to secure the necessary surface finish.
Recent improvements in the resolution of extrinsically addressed LCD's have led to the desirability of a further glass properties, namely, a high glass strain point, a low density, and a high modulus. Strain point is used as an indication of the thermal shrinkage of the glass. As can be appreciated, the lower the strain point, the greater is this thermal shrinkage. Low thermal shrinkage is desirable for precise alignment during successive photolithographic and other patterning steps during the TFT processing. Consequently, glasses having higher strain points are generally preferred for extrinsically addressed LCD's, particularly those which employ poly-Si TFT technology. Thus, there has been considerable research to develop glasses demonstrating high strain points so that thermal shrinkage is minimized during device processing. Corning Code 1737 glass, which has the highest strain point (666° C.) in the AMLCD substrate industry, is rapidly becoming an industry standard. Concurrent with their high strain points, these glasses often have high melting temperatures, e.g. on the order of 1550-1650° C. Low densities and high modulii are desired to minimize sag during thermal processing and to allow for thinner and lighter displays.
Another technology termed “chip-on-glass” (COG) has further emphasized the need for the substrate glass to closely match silicon in thermal expansion. Thus, the initial LCD devices did not have their driver chips mounted on the substrate glass. Instead, the silicon chips were mounted remotely and were connected to the LCD substrate circuitry with compliant or flexible wiring. As LCD device technology improved and as the devices became larger and required finer resolutions, these flexible mountings became unacceptable, both because of cost and of uncertain reliability. This situation led to Tape Automatic Bonding (TAB) of the silicon chips. In that process the silicon chips and electrical connections to the chips were mounted on a carrier tape, that subassembly was mounted directly on the LCD substrate, and thereafter the connection to the LCD circuitry was completed. TAB decreased cost while improving reliability and increasing the permitted density of the conductors to a pitch of approximately 200 μm—all significant factors. COG, however, provides further improvement over TAB with respect to those three factors. Hence, as the size and quality requirements of LCD devices increase, COG is demanded for those devices dependent upon the use of integrated circuit silicon chips. For that reason, the substrate glass preferably demonstrate a linear coefficient of thermal expansion closely matching that of silicon; i.e., a linear coefficient of thermal expansion (0°-300° C.) between about 32-39×10−7/° C.
It would therefore be desirable to provide a glass substrate having a CTE in the 30-40°×10−7/° C. range and a strain point greater than 650° C., more preferably greater than 675° C. A density less than 2.50 g/cm3 and a Young's modulus of greater than 11.0 Mpsi would also be desirable. It would also be desirable for the glass to be capable of being manufactured using the float process.